The present invention is related to the field of electrostaographic printing and especially to the field of printing, using liquid toner.
Modern liquid toner electrostatic imaging began with the invention of a new class of toners referred to herein as ElectroInk(copyright) (which is a trademark of Indigo, Nev. of The Netherlands). This toner is characterized by its comprising toner particles dispersed in a carrier liquid, where the toner particles are comprised of a core of a polymer with fibrous extensions extending from the core. When the toner particles are dispersed in the carrier liquid in a low concentration, the particles remain separate. When the toner develops an electrostatic image the concentration of toner particles increases and the fibrous extensions interlock. A large number of patents and patent applications are directed toward this type of toner and charge directors which are comprised in it. These include: U.S. Pat. Nos. 4,794,651; 4,842,974; 5,047,306; 5,407,307; 5,192,638; 5,208,130; 5,225,306; 5,264,312; 5,266,435; 5,286,593; 5,300,390; 5,346,796; 5,407,771; 5,554;476; 5,655,194; 5,792,584 and U.S. Pat. No. 5,5923,929, PCT Patent publication WO/92/17823 (equivalent to U.S. patent application Ser. No. 08/203,596) and PCT patent application PCT/IL99/00394, the disclosures of all of which are incorporated herein by reference.
It has been discovered that this type of toner allows for high quality offset printing at high speed. However, this type of printing is described inter alia in patents and patent application Nos. 4,678,317; 4,860,924; 4,980,259; 4,985,732; 5,028,964; 5,034,778; 5,047;808; 5,078,504; 5,117,263; 5,148,222; 5,157,238; 5,166,734; 5,208,130; 5,231,454; 5,255,058; 5,266,435; 5,268,687; 5,270,776; 5,276,492; 5,278,615; 5,280,326; 5,286,948; 5,289,238; 5,315,321; 5,335,054; 5,337,131; 5,376,491; 5,380,611; 5,426,491; 5,436,706; 5,497,222; 5,508,790; 5,527,652; 5,552,875; 5,555,185; 5,557,376; 5,558,970; 5,570,193; 5,571,645; 5,572,274; 5,585,900; 5,592,269; 5,596,396; 5,610,694; 5,636,349; 5,655,194; 5,737,666; 5,745,829; 5,749,032; 5,793,490; 5,854,960; 5,864,353; 5,900,003; 5,915,152; 5,923,929; 5,935,754 and PCT publications (now U.S. patent applications) WO 96/29633 (08/930,249); WO 96/35182 (08/945,415); WO 96/29633 (08/894,707); WO 97/07433 (09/011,634); WO 97/39385 (09/171,396); WO 98/55901 (09/445,035) and WO 99/454,433 and PCT applications PCT/IL98/00553 and PCT/IL99/00363, the disclosures of all of which are incorporated herein by reference. Systems incorporating various ones of these patents are sold under the names E-Print 1000(copyright), Omnius(copyright), Turbostream(trademark) and Cardpress(trademark).
In the course of time various methods for development of latent electrostatic images have been developed for liquid toner, as described in the above referenced patents and patent applications. In addition, the speed of printing has increased. Printing is performed on a large number of materials. Other operating conditions have also changed.
In general, Electroink comprises a polymer or polymers (usually pigmented) which solvate the carrier liquid at some temperature above room temperature (and preferably above normal storage temperatures of 30-40xc2x0 C.) and do not solvate the carrier liquid or dissolve substantial amounts of it below that temperature. Above the solvation temperature the polymer adsorbs the carrier liquid and is plasticized and softened by it. At elevated temperatures the toner material is thus soft enough to bond with a paper substrate. In practice, the temperature and pressure at which transfer to paper is made is controlled so that the transfer is complete, the transferred toner is fixed to the paper and the image is not squashed.
PCT publication WO 92/17823 (U.S. patent application Ser. No. 08/203,596) discloses a toner comprising two components in which the viscosity vs. temperature characteristics are specified. In particular, it was found that toner based on a mixture of polymers having a break-point on a semi-logarithmic plot of viscosity vs. temperature at between 3xc3x97105 and 5xc3x97106 at a temperature of below 65xc2x0 C., when in a 40% by weight of toner polymer mixed with carrier liquid, allowed for good transfer and fixing of the image at relatively low transfer blanket temperatures. This weight percentage was chosen as a standard since it represented a proportion believed to exist during transfer of the toner to a paper substrate.
An aspect of some preferred embodiments of the invention is concerned with liquid toners comprising pigmented toner particles in a carrier liquid. Preferably, the polymer in a 40% weight to weight mixture with carrier liquid, has a break point on a semi-logarithmic viscosity vs. temperature curve at between 3.8xc3x97103 centipoise and a value below 5.3xc3x97105 centipoise and a temperature of below 68xc2x0 C., more preferably below 65xc2x0 C. In preferred embodiments of the invention, the toner particle polymer is comprised of two or more polymers.
Preferably one of the polymers comprises a first, relatively higher viscosity material in an amount of between 70 and 90 percent and a second polymer having a second, relatively lower viscosity in an amount of between 5 and 25 percent. In some preferred embodiments of the invention, a third polymer having a relatively low viscosity is comprised in the toner polymer in an amount between 2 and 10 percent. In preferred embodiments of the invention, the third polymer is reactive with components of a substrate on which the toner is printed.
The present inventors have found that under conditions in which the amount of pigment in the toner is increased over that in prior art (and thus thinner layers of toner are utilized) and/or in which the amount of liquid is reduced by removing liquid from toner prior to its transfer, and/or in which the speed of the process is increased and the time available for transfer to the paper decreased, the optimum viscosity is reduced over that of toners of the prior art.
An aspect of some preferred embodiments of the invention relates to a toner including a polymer material that is reactive with a substrate on which it is printed.
In general, printing with toner materials on paper is relatively straightforward, especially when transfer thereto is by heat and pressure, since the toner is forced into the paper and forms a good bond with it.
Thus, in a preferred embodiment of the invention the polymers utilized include a minor proportion of a first polymer having a relatively greater reactive affinity for the paper and a major portion of a second polymer having substantially no reactive affinity or a relatively smaller reactive affinity of the paper. In preferred embodiments of the invention, the first polymer comprises between 2-10% of the total amount of polymer in the toner particles. In some preferred embodiments of the invention, the second polymer is a mixture of polymers.
In one preferred embodiment of the invention the first polymer has an anhydride functionality and is compatible (forms a homogeneous mixture) with the other toner polymers in the particles. The anhydride functionality is believed to form a bond, at the fixing temperature of about 70-90xc2x0 C., with the cellulose in the paper. In particular, it is believed that the oxygen in the anhydride bonds with hydrogen in the cellulose. Thus, under high speed printing conditions, the anhydride (or other material with a high affinity for paper) provides for adequate boding of the toner to the paper, despite the relatively shorter times available for transfer of the toner to the paper and for fusing it thereto.
The first polymer may be in the form of a terpolymer containing an anhydride functionality, such as maleic anhydride terpolymer or it may, for example, be maleic anhydride grafted linear low density polyethylene, maleic anhydride grafted polypropylene copolymer, maleic anhydride grafted linear ethylene acetate polymer. Other materials with anhydride functionality may also be used.
Other minor portion polymers believed to work in a similar manner include polymers having an epoxy functionality. It is believed that when activated by water or hydroxyl functionalities in the paper, the epoxy functionality bonds with hydroxyl functionalities in the paper.
While the second polymer may include any polymer or mixture of polymers having suitable viscosity, solvation and other parameters for toner, some especially suitable first polymer materials include, ethylene methacrylic acid copolymers and their ionomers, ethylene acrylic acid copolymers and their ionomers, polyamides, etc., or mixtures thereof It should be understood that some of these materials may have higher or low than optimal viscosity and that blends of the polymers may be required to provided a desired viscosity.
There is thus provided, in accordance with a preferred embodiment of the invention, a liquid toner comprising:
a carrier liquid; and
toner particles dispersed in the carrier liquid, said toner particles comprising:
a polymer blend comprising:
a first polymer comprising a minor portion of said blend and having a relatively higher chemical reactivity with paper; and
a second polymer comprising a major portion of said blend and having a relatively lower or null chemical reactivity with paper.
Preferably, the polymer blend has, on a semi-logarithmic viscosity vs. temperature cooling curve, a transition at a temperature below about 65xc2x0 C., wherein at temperatures below the transition temperature, the rate of change of viscosity with temperature is higher than the rate of change at temperatures above the transition temperature.
Preferably, at the transition temperature, the viscosity is below about 107 centipoise.
Preferably, at the transition temperature, the viscosity is below about 2xc3x97105 centipoise.
Preferably, at the transition temperature, the viscosity is above about 104 centipoise.
In a preferred embodiment of the invention the second polymer comprises at least two polymers.
In a preferred embodiment of the invention, the polymer blend is substantially insoluble in the carrier liquid and wherein at least one of the polymers solvates the carrier liquid at an elevated temperature.
In a preferred embodiment of the invention, the proportion of the first polymer in the blend is between about 2% and about 10%, preferably about 5%. In a preferred embodiment of the invention, the first polymer comprises a polymer having an anhydride functionality. The first polymer may include, in accordance with various preferred embodiments of the invention, one or more of maleic anhydride terpolymer, maleic anhydride grafted linear low density polyethylene, maleic anhydride grafted polypropylene copolymer, and maleic anhydride grafted linear ethylene acetate polymer.
In preferred embodiments of the invention, the second polymer comprises one or more of ethylene methacrylic acid copolymer; an ionomer of ethylene methacrylic acid copolymer, an ester of ethylene methacrylic acid copolymer, low molecular weight ethylene acrylic acid copolymer, an ionomer of low molecular weight ethylene acrylic acid copolymer, an ester of ethylene acrylic acid copolymer, and an acid modified ethylene vinyl acetate terpolymer.
Preferably, the toner particles comprise at least one pigment.
There is further provided, in accordance with a preferred embodiment of the invention, a liquid toner comprising:
a carrier liquid; and
toner particles dispersed in the carrier liquid, said toner particles comprising:
a polymer blend comprising at least first and second distinct polymers, wherein the polymer blend has, on a semi-logarithmic viscosity vs. temperature cooling curve, a transition at a temperature below about 65xc2x0 C., wherein at temperatures below the transition temperature, the rate of change of viscosity with temperature is higher than the rate of change at temperatures above the transition temperature.
In a preferred embodiment of the invention, the transition temperature, the viscosity is below about 107 centipoise. Preferably, at the transition temperature, the viscosity is below about 2xc3x97105 centipoise.
In a preferred embodiment of the invention, at the transition temperature, the viscosity is above about 104 centipoise.
In a preferred embodiment of the invention, the polymer blend is substantially insoluble in the carrier liquid and wherein at least one of the polymers solvates the carrier liquid at an elevated temperature.
In a preferred embodiment of the invention, the polymer blend comprises one or more of ethylene methacrylic acid copolymer, an ionomer of ethylene methacrylic acid copolymer, an ester of ethylene methacrylic acid copolymer, low molecular weight ethylene acrylic acid copolymer, an ionomer of low molecular weight ethylene acrylic acid copolymer, an ester of ethylene acrylic acid copolymer, or an acid modified ethylene vinyl acetate terpolymer.
Preferably, the toner particles comprise at least one pigment.
There is further provided, in accordance with a preferred embodiment of the invention, a method of printing comprising:
providing an electrostatic image; and
developing the electrostatic image with a toner in accordance with any of the preceding claims.
Preferably, the method includes transferring the developed image from a surface on which it is developed to a final substrate.
Preferably, the final substrate contains cellulose.
In a preferred embodiment of the invention, the developed image comprises first transferring the image to an intermediate transfer member and then transferring the image therefrom to the final substrate.
In a preferred embodiment of the invention, transferring comprises fixing the transferred image to the final substrate.